Inter alia, luminous points comprising a light source, in particular an LED, have been used heretofore for display boards outdoors which have to display variable contents, such as, for instance, so-called variable-message signs for influencing traffic. By means of electronic driving of the LEDs, luminous points can be switched on and off, and also represent gradual differences in brightness. These luminous points are either arranged in different light colors according to symbols to be represented such as traffic signs, for instance, or used over relatively large areas in a grid arrangement in order thus to be able to represent arbitrarily programmable graphics, texts or even images. The luminous points here function as so-called pixels.
In contrast to LED-based large-area video screens, which require a wide light emission for an audience situated in front of them, traffic representations are limited to much narrower emission or viewing angles since often they have to be viewed from a large distance only from one traffic lane. Moreover, they generally show constant displays and still images which change only infrequently. This results in considerable simplifications in terms of the driving electronics and in terms of the energy consumption, and hence much lower procurement and operating costs. However, greater viewing distances and weather influences also require higher light intensities; specific lighting requirements are also prescribed.
Whereas heretofore single-colored luminous points have usually been sufficient, and the small number of image representations have been implemented for instance from a combination of in each case one red, green and blue light point, combined as a color pixel, in a grid arrangement, in the future it is to be expected that more and more color representations with ever higher resolution will be desired and a pixel arrangement comprising three single-colored luminous points would be too large and too expensive.
In comparison therewith, in the case of LED large-format video screens, LEDs produced specifically therefor are used which contain the three primary colors red, green and blue in the form of three LED crystals in a common housing, wherein each color can be driven individually. In this case, the three colors have an identical emission characteristic achieved, for instance, by admixing scattering means into the LEDs. These so-called full-color LEDs or multi-LEDs have a planar light exit surface and are arranged in the pixel group. They emit their light as so-called cosine emitters, wherein the light is the most intense in the center and up to the edge at 90° decreases to zero gradually according to the function of a cosine curve. Since the light radiates hemispherically into a very large region, the brightness is rather low even in the center and moreover cannot be increased owing to the increasing outlay in respect of energy and cooling, for which reason such screens are only used indoors. Outdoors, large-format video screens are likewise implemented with specific, individual red, green and blue LEDs with an integrated, light-focusing lens dome and an oval design and light emission, since otherwise the necessary daylight brightness cannot be achieved economically.
In the case of all LED large screens, it is particularly important for the light emission of the LEDs to be implemented as identically as possible, since otherwise color shifts, color fringes, or color spots appear in the case of lateral viewing. It is very complicated to install individual single-colored LEDs with a lens dome such that they are all precisely aligned with one another, particularly if the LEDs still stand on wire feet.
The full-color LEDs or multi-LEDs are soldered as SMD design in a simple manner in the grid onto a suitable printed circuit board, which also results in an accurate level alignment; the light emission of the three LED crystals is already of an identical type.
It is then obvious to use the simple and cost-effective design of a full-color LED or multi-LED for high light powers as well, by means of the widely emitting light being focused in a simple manner by a convergent lens placed in front. However, this intention fails owing to the fact that the light from the three color crystals cannot be focused identically to one another by the lens placed in front, but rather each color aims in a different direction, in accordance with the arrangement of the crystals in the LED and the optical laws of imaging. The colors therefore have to be mixed well prior to focusing.
There are already proposals for the color mixing of different LEDs. JP 2008 047482 A (Epson) presents display lighting by means of edge irradiation with different-colored LEDs, polarization filter and color mixing. Here the objective is fundamentally different; after all, the color mixing without focusing of the light already arises in the multi-LEDs themselves.
It is also the case in the known advertising boards with edge irradiation by LEDs in different colors that very good color mixing without focusing arises automatically by virtue of the light from each LED being distributed over the entire display area by multiple reflection and scattering.
Room luminaires having red, green and blue LEDs also generate a uniformly mixed white, provided that they use LEDs having an identical emission characteristic and furthermore make them more uniform by additional scattering by means of structured cover plates, thus giving rise to a uniform light effect and color.
Color mixing without light focusing can thus be effected in a simple manner by means of light scattering. Mention should also be made of status displays on electronic devices, which use LEDs having a plurality of crystals which have for instance red and green and, as mixed color, yellow.
The light from these LEDs is guided by means of light guides to the housing surface and emitted there with wide scattering. Scattering also means, in principle, a loss of energy of the light, which is manifested in a reduced focusing capability, and also losses owing to light which, as a result of scattering, prematurely leaves the optical system and cannot be utilized.
However, an arrangement for focused mixed light is already known as well. US 2010 020565 A1 (Seward) proposes completely mixing the different-colored light from the LEDs in a small (Ulbricht) sphere and guiding it through a light guide rod into a highly focusing convergent optical system. In practice, the proposal fails owing to the high scattered-light losses at the walls of the sphere, and the required structural size and the outlay for this arrangement.
In principle, light transmission by means of a light guide, at one end of which a light source introduces radiation and at the other end of which the light being emitted is distributed by an optical system, has already been known for a long time. However, this basic system can be modified by an enormously large number of parameters, such that an immense diversity of qualities and design possibilities arises.
If the properties of a light guide are considered, then it is generally assumed, besides many other properties, that said light guide mixes the light “per se” and therefore functions, in principle, as light color or intensity mixer, such as, for instance, a rough surface or a translucent, diffuse material, such as milk glass, for example. In actual fact, a light guide is a highly transparent, thoroughly precise optical element which, in terms of its function, is no different from an optical lens, an optical prism or other optical objects. In its interior, a precisely determinable beam path takes place which depends only on the type of light source and the effect thereof on the entrance surface.
The impression of a “mixing” property arises by virtue of the fact that the light is forwarded by multiple total reflection at the light guide walls and therefore surface tolerances have a very high influence on the result, since even tiny angular errors of the wall surfaces are doubled in the course of total reflection and the “series connection” of the numerous reflections leads to further tolerances of the light deflection. Therefore, long light guides actually have a mixing effect that arises as a result of unavoidable manufacturing tolerances. However, short light guides manufactured with very high precision have no mixing effect whatsoever, as a result of which they act on the light like optical lenses or prisms.
Another criterion is the efficiency of the system light source-light guide-distribution. If virtually every light ray from the light source is guided through the light guide and emitted usefully, then the efficiency is almost 100%. In practice, not all of the light rays pass into the optical waveguide; they miss the entrance surface or are reflected back at the latter. The materials and surfaces also absorb part of the light, and in the course of distribution many light rays will also radiate into regions not required. In particular, fiber-bundle light guides have so-called “interstitial losses” between the round fibers, in which case although light radiates forth, it is not forwarded. Likewise, the cladding layer of a fiber composed of low refractive index material is also unable to forward light.
A further criterion is the optical effect of a light guide. If the light guide widens, then the light is focused since the axial angle of each light ray is reduced upon each reflection at the wall. If it narrows, however, the light is scattered, in which case the “aperture” of the light guide is exceeded very rapidly. The light then impinges on the reflection walls at angles that become steeper and steeper, until the light can emerge laterally from the light guide and is lost.
EP 0 596 865 A2 discloses a device for emitting light, wherein an optical element and as light source an LED are used. In that case, it should be taken into consideration that glass-fiber optical waveguides are used for transmitting the light, and they can also have virtually arbitrary bends. The emission angle of the overall light beam is influenced by selectively switching on different LEDs. Furthermore, said glass-fiber optical waveguides, preferably fiber bundles, considered by themselves, each have a circular cross section.
US 2009/0052189 A1 discloses an arrangement for producing a spotlight with high focusing and simultaneous mixing of the primary colors (R, G, B). This LED emitter comprises an LED light source having a plurality of LED crystals, a rod lens having the function of a light guide rod, and an optical element. To that effect, US 2009/0052189 A1 discloses a known basic arrangement, supplemented by a “first” focusing optical system, which focuses the light emitted by the LED light source onto the entrance surface of a square light guide rod, which tapers conically and the exit surface of which is situated at the focus of a reflector. In that case, the primary convergent lens is arranged in front of the light guide rod and the light of the LED is focused onto the entrance surface thereof; furthermore, the rod lens has a tapering cross-sectional form from the LED light source to the lens. Consequently, in US 2009/0052189 A1, a reflector is primarily used since the conically tapering light guide rod emits the light into a hemisphere, which can be focused more easily by a reflector. However, in the case of the second convergent system presented in US 2009/0052189 A1, by means of dichroic mirrors, the light is already premixed. These essential differences are substantiated in the objective of US 2009/0052189 A1 to achieve a maximum focusing, while a specific light distribution is intended to be achieved with the present invention.
By contrast, WO 2006/054199 A1 discloses a light source comprising a light engine having at least one LED and/or at least one laser light emitting element, for generating and coupling in light, in at least one light guide comprising at least one coupling-out element for coupling out the light.